Stents with proximal and distal end elevations

ABSTRACT

A prosthetic stent with a tubular wall having local inwardly or outwardly formed elevations. Stents having such elevations have a higher mechanical stability if bent according to the curvature of the body vessels to be supported or repaired. Also a method for manufacturing a stent with such elevations is described.

This is a divisional of prior application Ser. No. 09/874,609, (now U.S.Pat. No. 6,652,577), filed Jun. 5, 2001 as a divisional of applicationSer. No. 09/431,988 (now U.S. Pat. No. 6,240,978), filed Nov. 2, 1999 asa divisional of application Ser. No. 08/993,033 (now U.S. Pat. No.5,993,483), filed Dec. 18, 1997.

BACKGROUND OF THE INVENTION

This application claims priority under 35 U.S.C. § 119 of EuropeanPatent Application No. 97202152.1, filed in the European Patent Officeon Jul. 17, 1997.

The present invention relates to a stent for use in a body passageway,comprising a flexible self-expanding braided tubular wall being composedof helically wound wires and having proximal and distal ends. Theinvention also relates to a method for manufacturing such a stent.

A stent of the type as mentioned in the introduction is described forexample in U.S. Pat. No. 4,655,771. The tubular wall is composed ofseveral flexible thread elements each of which extends along a helixwith the center line of the tubular wall as a common axis. The threadelements are arranged in two groups of opposite directions of windingcrossing each other in a way to form a braided configuration. This is toimpart to the tubular body the necessary stability for supporting avessel. The diameter of the tubular wall can be changed by axialmovement of the ends relative to each other. The stent is transluminallyinserted into position in its radially compressed state and thensubjected to expansion staying in place by a permanent pressure againstthe inner wall of the body passageway. The stability of the tubular bodydepends in general from the number of the thread elements, theirdiameter and material and from the braiding angle of the thread elementsat their crossings. It is preferred to have the axially directedbraiding angle being obtuse, i.e. larger than 90°, in order to obtain alarge force in radial directions. But the braiding angle also influencesthe shortening of the stent, which is the reduction of the scent lengthupon conversion from its compressed to its expanded state. At a givendiameter expansion the stent shortens less at braiding angles smallerthan around 120° than at larger angles.

In the following stents with a braiding angle larger than about 120° arereferred to as “normal-shortening” whereas stents having a braidingangle of less than about 120° are referred to as “less-shortening.” Itis an advantage of less-shortening stents that they can be placed moreaccurately because the practitioner can better estimate the finalpositions of the stent ends after expansion. The less-shortening featurecomes also to fruition when the stent is implanted in a moving holloworgan in which the stent is repeatedly radially compressed, such as inthe esophagus, in the trachea or in a pulsating blood vessel. In thosecases the reduced shortening of the stent is less traumatic for theinner wall of the hollow organ since the stent ends perform smalleraxial movements than normal-shortening stents do. For the aforesaidreasons less-shortening stents are preferably implanted in ostiumregions, for example in the aorta next to the entries into the renalarteries or in side branches. Exact placement capability and less axialmovement of the stent ends reduce the risk of unwanted perturbation orobstruction of the blood flow by stent ends projecting into the ostium.

However, stents of the less-shortening type comprise smaller hoopstrength compared to normal-shortening prostheses due to their smallerbraiding angle. A consequence of the lower radial force is a reductionof the self-fixation characteristics with the risk of a local axialdisplacement of the stent within the body passageway. Moreover, thestent is not stable enough to resist flattening if it is implanted inarched vessels. This means that a more or less strong deformation of thestent cross-section deviating from its original circular shape canpartially close the stent.

In EP-A-O 775 471 an improved stent is disclosed comprising a flexibleself-expanding braided tubular wall having a proximal segment of smallerdiameter and a distal segment of larger diameter and in-between anintermediate segment forming a truncated cone. A covering layer isarranged within the tubular wall. Although the document does notdisclose any specific braiding angles the proximal segment will have asimilar braiding angle as the above described less-shortening stent andthe distal segment will have a larger braiding angle. The differentgeometry can be derived from the manufacturing methods as described inthe document. The large-diameter segment serves as a migration anchorwhile the less-shortening segment with smaller diameter makes an easierand safer way through curves or at the end of for example a food pipe.But the less-shortening stent segment still has not sufficient shapestability for use in curved areas of body vessels. The cross-section ofthis segment may be deformed elliptically if bended in curved bodyvessels as it will occur generally for less-shortening stents. Moreover,because of the conical shape such a stent can be used only at particularareas, such as in food pipes. In addition, it is to be said that theused manufacturing methods are quite expensive.

All documents cited herein, including the foregoing, are incorporatedherein by reference in their entireties for all purposes.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to improve aless-shortening stent such that it can be used universally, and morespecifically in moving and/or in curved body passageways avoidingmigration and flattening deformation thereof. A further object of theinvention is to provide a stent which can be manufactured easier.

The term “elevation” has the meaning of an impression or bulge of thestent wall as well in the negative as in the positive sense, i.e.extending inwardly or outwardly of the tubular stent wall. Accordingly,the tubular wall has at least a local inwardly and/or outwardly formedelevation, whereby the wires are plastically deformed in a way that thenumber of degrees of freedom for their movement within the braiding isreduced. This means that the mesh cells defined by the braided wires are“frozen” by a reduced capability of the wires to rotate and shiftrelative to each other at their crossing points. The braided tubularwall retains its less-shortening feature and becomes more stable againstradial deformation. A further advantage of the formed elevations is thepossibility to make a short stent of the type mentioned in theintroduction. Such stents are usually cut from the braiding blank andcomprise an unwanted conical shape due to a memory effect from thebraiding process. This shape can be converted into a cylindrical tubeand conserved by forming elevations on the stent wall.

Where the elevations are distributed regularly over the tubular wall,the stent will be anchored firmly with the tissue of the body vesselwithout damaging. The homogeneity of the elevation distribution is forexample preferred if the stent is to be implanted in a curved area of abody passageway.

More dense distribution of the elevations at the proximal and distalends of the stent will provide higher stability at these areas forbetter anchoring thereof with the tissue of the body vessel. Thisembodiment is preferred if the stent is to be implanted in ostiumpositions for a safe fixation of the stent ends in order to preventmigration of the stent and disturbing for example the blood flow into aside branch through this ostium. Another preferred application of such astent is the support of a vessel having a hard plaque stenosis wherebythe stent comprises a higher density of elevations in the stenoticregion.

In a preferred embodiment of the invention the elevations are formedoutwardly so that they can serve as an anchor against stent migration byengaging into the inner vessel wall to be supported. Moreover, thedeployment of such a stent with delivery devices as known in the art isenhanced since the retraction of the outer sheath is easier. Thisresults from a reduced friction between the inside of the deliverysheath and the radially outwardly pressing stent touching the sheathonly at the elevations.

In another preferred embodiment of the present invention the localelevations have an elongate shape which makes the manufacturing of suchstents very easy by using wires to emboss the tubular wall. Theelevations may have an arched cross-sectional shape. Preferably theheight of the elevations are approximately one to two times the wirediameter of the braid.

These embossments or elevations can be formed in patterns helically onthe tubular wall, where in a preferred embodiment the helical elevationpattern has a different pitch than the wires of the braid in order todeform as many wires as possible. The elevations may also be formedannularly or in an axial direction on the tubular wall depending on thedesired effect. Where the elevations are placed annularly the stent wallcomprises an improved radial stability, whereas elevations in axialdirections impart to the stent a higher longitudinal stability which isespecially useful for implantation in the airways.

The manufacturing method according to the present invention isdetermined by the steps of forming an elongate mandrel having at leastone local outwardly bound elevation, forming an elongated tubular braidof spring steel having proximal and distal ends and an inner diametercommensurate with the diameter of the mandrel, engaging said tubularbraid over said mandrel, heating the tubular braid on the mandrel,cooling the tubular braid and disengaging the braid from the mandrel.Preferably previous to the disengaging step the braid will be compressedin the axial direction.

In sum the present invention relates to a stent for use in a bodypassageway. A flexible self-expanding braided tubular wall is composedof helically wound wires and has proximal and distal ends, wherein thetubular wall has at least a local inwardly and/or outwardly formedelevation. The local elevations may be distributed regularly over thetubular wall and distributed more densely at the proximal and distalends. The local elevations of the stent may be formed outwardly and mayhave an elongated shape. The stent elevations may have an archedcross-sectional shape and/or a height of approximately one to two timesof the diameter of the wires. The elevations may be formed helically onthe tubular wall. The helical elevation may have a different pitch thanthe wires of the braid. The elevation may be formed annularly on thetubular wall or formed in an axial direction on the tubular wall.

The invention further relates to a method for manufacturing a stent byforming or providing an elongated mandrel having at least one localoutwardly bound elevation; forming or providing an elongated tubularbraid of spring steel having proximal and distal ends and an innerdiameter commensurate with the diameter of the mandrel; engaging thetubular braid over the mandrel; heating the tubular braid over themandrel; cooling the tubular braid; and disengaging the braid from themandrel. Prior to disengaging the braid from the mandrel, the braid maybe compressed in an axial direction. The steps of heating the tubularbraid over the mandrel and cooling the tubular braid may be performedunder vacuum condition.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will become readily apparent from the subsequent description,wherein the invention will be explained in further details withreference to the accompanying drawings which show, diagrammatically andby way of example only, preferred but still illustrative embodiments ofthe invention.

FIG. 1 shows a stent with a helical elevation in side view,

FIG. 2 shows a cross-sectional view according to line A-A in FIG. 1,

FIG. 3 shows a stent with a plurality of radial elevations in side view,

FIG. 4 shows a stent with a plurality of axial elevations in side view,

FIG. 5 shows the stent of FIG. 4 in front view according to arrow B,

FIG. 6 shows a stent similar to that in FIG. 1, but with increaseddensities of elevations at its ends, and

FIG. 7 shows a stent similar to that in FIG. 3, but with increaseddensities of elevations at its ends.

In the following description of the drawings the same reference numbershave been used for all figures if not mentioned otherwise.

DETAILED DESCRIPTION OF THE INVENTION

The stent depicted in FIG. 1 comprises a flexible self-expanding braidedtubular wall 1 which is composed of a first plurality of parallel springstainless steel wires 2 helically wound in a first direction crossing asecond plurality of parallel spring stainless steel wires 3 helicallywound in a second direction opposite to the first one. The braidedstructure assures contraction of the stent in the radial direction whenthe proximal and distal ends 4 and 5 of the stent are pulled away fromone another as exemplified by arrows 6, and self-expansion of the stentin the radial direction when the pull according to arrows 6 is released.This configuration is well known in the art and needs no furtherexplanation. Of course, other known braidings or patterns providing thesame effect may be used.

The tubular wall 1 of the stent having a helical pattern of elevations 7which is outwardly formed and has an angle of gradient or pitch slightlysmaller than the angle of gradient or pitch of the steel wires 2 shownin the same winding direction. The elevations 7 have an elongate andarched cross-sectional shape. The height of the elevations 7 over thetubular wall 1 is about once or twice the diameter of the wires 2 or 3of the braided configuration. The wires 2 and 3 may be made of ametallic material, e.g. stainless steel, which may be filled with aradiopaque core, or made of a thermoplastic polymer, such as polyesters,polyurethanes, polycarbonates, polysulphides, polypropylene,polyethylene or polysulphonates. Normally the diameter of the wires 2and 3 lie within the range 0.01 to 0.5 mms. The helical elevation 7provides a greater stability of the meshes of the braided tubular wall1, i.e. the parallel wires 2 and the parallel wires 3 will be preventedfrom moving apart at the crossing points 8. Especially in thecross-sectional view of FIG. 2 it can be seen that wires 2 and 3 havebeen deformed locally in a tubular shape. The elevation pattern isnormally distributed in a regular manner over the tubular wall 1.Therefore a specific wire 2 or 3 will have several elevation areas overits whole length within the tubular wall 1 and a much greater stabilityof the wires 2 and 3 within the braid will be obtained. The elevation isfurther smooth curved, i.e. having a continuous smoothly inclining anddeclining curvature with the effect that the spring activity of thewires 2 and 3 will be reduced in the areas of the elevations. On theother hand the braiding angle between the wires 2 and 3 will be enlargedlocally in the area of the elevations which will additionally enhancethe mechanical stability of the tubular wall 1. In fact, the meshes areimmobilized or “frozen” at the crossing points of the wires 2 and 3 inthe area of the elevation. By the frozen meshes the tubular wall 1 willobtain an enlarged shape stability which will resist the deformingforces of the body vessel. The elevation 7 will also reduce the tendencyof the wires 2 and 3 to debraid at the proximal and distal ends 4 and 5of the tubular wall 1. Thus the aforementioned stent will have a greaterform or shape stability if the tubular wall 1 will be bent in bloodvessels with a strong curvature, i.e. the circular cross-section of thetubular wall 1 will be retained and not deformed to an elliptical one ascan be observed with less-shortening stents.

Another possibility of providing elevations for stents according to thepresent invention is shown in FIG. 3, where the stent having annularpattern of outwardly formed elevations 12 which, are equidistant andparallel to each other. Here also the stability of the stent has beenimproved over the well-known stents. If an annular pattern of elevations12 will be provided near the proximal and distal ends 4 and 5 thetendency of debraiding of the wires 2 and 3 can be reduced further.

In FIG. 4 another example of a stent according to the invention isshown, wherein outwardly formed elevations 13 are provided in an axialdirection on the tubular wall 1, which elevations 13 are alsoequidistant and parallel to each other. The front view of FIG. 5 showsthat these elevations are also smoothly curved as in the previousexamples. Since the wires 2 and 3 are intertwined with a relativelydense mesh the four patterns of elevations 13 as depicted in thisexample are sufficient to prevent debraiding at the proximal and distalends 4 and 5 of the stent.

Although the elevations 7, 12 and 13 in the examples of FIGS. 1, 3 and 4are formed outwardly on the tubular wall 1, they may also be formedinwardly on the tubular wall 1 or possibly provided in combination ofoutwardly and inwardly formed elevations.

As mentioned previously, more dense distributions of elevations at theproximal and distal ends of the stent will provide higher stability atthese areas for better anchoring of the stent with the tissue of thebody vessel. Also, in connection with FIG. 3 it is noted above that anannular elevation pattern 12 near the proximal and distal ends 4 and 5can reduce the debraiding tendency. FIG. 6 shows a stent of the typeshown in FIG. 1, but with increased densities of elevations at theproximal and distal ends. FIG. 7 shows a stent of the type shown in FIG.3, but with annular elevation patterns near the proximal and distal ends4 and 5.

The manufacturing of the aforementioned stents is as follows:

Firstly the stent will be produced in the known manner, i.e. the wires 2and 3 will be intertwined with a predetermined braiding angle and with apredetermined mesh size dependent from the wire cross-section. Thebraiding angle of the so formed stent will normally be between 100° and120°. Thereafter the stent will be pushed over a cylindrical mandrelwith a regular pattern of outwardly formed elevations like the helicalshape of wires provided on the surface of the mandrel as will be used toform a stent according to FIG. 1. The mandrel with the stent will thenbe heated up to process temperature, kept under process temperature fora certain period of time, and cooled down afterwards. The heating andcooling procedure is carried out under vacuum condition. In the case ofstainless steel wires the thermal treatment may take up to sixteenhours, whereby the process temperature of 550° C. is maintained forabout two hours. Then the stent will be pulled from the mandrel. Incases where the patterns of elevations are not axially directed as forthe stent depicted in FIG. 4, the tubular wall 1 may be compressed inorder to enlarge the diameter thereof for an easier disengagement. Incase of the helical pattern of the elevations the stent may also beunscrewed from the mandrel.

Although other patterns of elevations may also be used for the stentsaccording to the invention the shown patterns are preferred since theyguarantee a smooth outer surface of the tubular wall 1 which isespecially important for stents to be used at delicate areas such asblood vessels in order not to damage the tissue. The helical shape andthe annular shape of the pattern of elevations are preferred for stentsused at the junction between the esophagus and the stomach as these willprevent much better the migration of the stent as in case of the axialpattern of elevations. In particular the elevations may also be formedinwardly instead of outwardly as shown and described above, i.e. thetubular stent wall having depressions. This may be advantageous if thebody vessel to be repaired needs more support and a larger contact areawith the stent.

Stents according to the present invention have a further advantage inthat they can be handled easier in the flexible shaft of the positioninginstrument since the friction between the stent and the inner wallthereof will be reduced. This applies more for the outwardly formedelevations as for the ones inwardly formed. But in both cases thefriction will be reduced in comparison to conventional stents. Thusrepositioning of stents with elevations as shown before has beenimproved also.

The above-described embodiments of the invention are merely descriptiveof its principles and are not to be considered limiting. Furthermodifications of the invention herein disclosed will occur to thoseskilled in the respective arts and all such modifications are deemed tobe within the scope of the invention as defined by the following claims.

1. A body insertable prosthesis, including: a flexible self-expandingtubular mesh wall comprising a plurality of elongate wire segmentscooperating to form multiple crossing points at which different ones ofthe elongate wire segments cross each other; wherein at a number ofselected crossing points, said number being less than the total numberof crossing points, pairs of the elongate wire segments crossing oneanother are shaped to form respective first and second elevationsextended in the same direction radially away from the tubular mesh wall;and wherein the elevations are arranged in a helical elevation patternon the tubular mesh wall.
 2. The prosthesis of claim 1 wherein: saidelevations extend radially outwardly from the braided tubular wall. 3.The prosthesis of claim 1 wherein: the plurality of elongate wiresegments include a first wire segment wound helically at a first pitchand a second wire segment wound helically at a second pitch differentfrom the first pitch, and the at least one elevation pattern has a thirdpitch different from the first pitch and different from the secondpitch.
 4. The prosthesis of claim 1 wherein: the tubular mesh wallcomprises at least one wire wound to form said crossing points, and saidelongate wire segments comprise different length-portions of the atleast one wire.
 5. A stent insertable into the body passageway,including: a flexible self-expanding braided tubular wall comprising atleast one first wire helically wound at a substantially constant firstpitch and at least one second wire helically wound at a substantiallyconstant second pitch different from the first pitch whereby the firstand second wires cooperate to form multiple crossing points of the atleast one first wire and the at least one second wire; wherein atselected crossing points, each of the first wire and the second wire isshaped to form an elevation extended away from the braided tubular wallin a selected direction radially of the braided tubular wall; andwherein said elevations are arranged in at least one elevation patternon the braided tubular wall, and the at least one elevation pattern hasa third pitch different from the first pitch and different from thesecond pitch; wherein the elevations are arranged in a helical elevationpattern on the braided tubular wall.